Light emitting device having a plurality of light emitting cells connected in series and method of fabricating the same

ABSTRACT

Disclosed are a light emitting device having a plurality of light emitting cells connected in series and a method of fabricating the same. The light emitting device includes a buffer layer formed on a substrate. A plurality of rod-shaped light emitting cells are located on the buffer layer to be spaced apart from one another. Each of the light emitting cells has an n-layer, an active layer and a p-layer. Meanwhile, wires connect the spaced light emitting cells in series or parallel. Accordingly, arrays of the light emitting cells connected in series are connected to be driven by currents flowing in opposite directions. Thus, there is provided a light emitting device that can be directly driven by an AC power source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/813,812, filed Jul. 12, 2007, which is a National Stage Applicationof International Application No. PCT/KR2005/002115, filed Jul. 4, 2005,and claims priority from and the benefit of Korean Patent ApplicationNo. 10-2005-0007310, filed on Jan. 26, 2005, which are herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device, and moreparticularly, to a light emitting device having a plurality of lightemitting cells, wherein the plurality of light emitting cells arearranged in serial arrays on a single substrate so as to be directlydriven using an AC power source.

2. Discussion of the Background

A light emitting diode is a electroluminescence device having astructure in that an n-type semiconductor of which major carriers areelectrons and a p-type semiconductor of which major carriers are holesare joined together, and emits predetermined light through recombinationof these electrons and holes. Such light emitting diodes are used asdisplay devices and backlights, and their application area has expandedto general illumination while substituting the conventional incandescentbulbs and fluorescent lamps.

A light emitting diode consumes less electric power and has a longerservice life as compared with conventional light bulbs or fluorescentlamps. The electric power consumption of a light emitting diode is lessthan a few tenths to a few hundredths of that of conventionalillumination devices, and the life span thereof is several to severalten times, thereby having reduced electric power consumption andexcellent durability.

However, the light emitting diode is repeatedly turned on and offdepending on the direction of a current. Therefore, if the lightemitting diode is used while being connected directly to an AC powersource, there is a problem in that the light emitting diode may beeasily damaged. Accordingly, it is difficult to use a light emittingdiode for the purpose of general illumination by connecting it directlyto a household AC power source.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light emitting devicethat comprises light emitting diodes, which can be connected directly toand driven by an AC power source.

Another object of the present invention is to provide a method offabricating a light emitting device that can be connected directly toand driven by an AC power source.

According to an aspect of the present invention for achieving theobject, there is provided a light emitting device having a plurality oflight emitting cells connected in series. The light emitting deviceaccording to the aspect of the present invention includes a buffer layerformed on a substrate. A plurality of rod-shaped light emitting cellsare located on the buffer layer to be spaced apart from one another.Each of the plurality of light emitting cells has an p-layer, an activelayer and a p-layer. Meanwhile, wires connect the spaced light emittingcells in series or parallel. Accordingly, arrays of the light emittingcells connected in series can be connected to be driven by currentsflowing in opposite directions. Thus, there is provided a light emittingdevice that can be directly driven by an AC power source.

The light emitting device of the present invention has a plurality oflight emitting diodes on a single substrate. Thus, the term “lightemitting cell” means each of the plurality of light emitting diodesformed on the substrate. Further, the term “array of serial lightemitting cells” means a structure having a plurality of light emittingcells connected in series.

Meanwhile, electrodes may be interposed between the wires and thep-layers. The electrodes form ohmic contacts with the p-layers, therebyreducing junction resistances.

According to another aspect of the present invention for achieving theanother object, there is provided a method of fabricating a lightemitting device having a plurality of light emitting cells connected inseries. The fabricating method according to the other aspect of thepresent invention comprises the step of forming a buffer layer on asubstrate. A plurality of laminated rod-shaped light emitting cells eachof which has an n-layer, an active layer and a p-layer sequentiallyformed are formed on the buffer layer. Thereafter, the p-layers and theactive layers are partially etched in sequence until the n-layers arepartially exposed. As a result, a plurality of light emitting cells areformed. In addition, wires for connecting the exposed n-layers of lightemitting cells and the etched p-layers of adjacent light emitting cellsare formed to connect the plurality of light emitting cells in series orparallel. According to the another aspect of the present invention,there is provided a light emitting device having a plurality of lightemitting cells connected in series, which can be connected directly toand driven by an AC power source. Further, since laminated rods spacedapart from one another can be formed on the buffer layer, an etchingprocess for separating the light emitting cells can be eliminated.

Meanwhile, electrodes may be formed on the p-layers after forming theinsulating film. The electrodes are in ohmic contacts with the P layers,thereby reducing junction resistances.

According to the embodiments of the present invention, there is provideda light emitting device that comprises light emitting diodes, which canbe connected directly to and driven by an AC power source. Therefore,the light emitting device can be used for the purpose of generalillumination using a household AC power source. Further, there isprovided a method of fabricating the light emitting device that can beconnected directly to and driven by the AC power source. According tothis method, it is possible to eliminate an etching process forseparating the light emitting cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating an operational principle of alight emitting device having a plurality of light emitting cellsconnected in series according to an embodiment of the present invention.

FIG. 2 is a flow chart illustrating a method of fabricating a lightemitting device having a plurality of light emitting cells connected inseries according to an embodiment of the present invention.

FIGS. 3 to 7 are sectional views illustrating the method of fabricatingthe light emitting device having the plurality of light emitting cellsconnected in series according to the embodiment of the presentinvention, wherein FIG. 4 is a perspective view of FIG. 3.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

The following embodiments are provided only for illustrative purposes sothat those skilled in the art can fully understand the spirit of thepresent invention. Therefore, the present invention is not limited tothe following embodiments but may be implemented in other forms. In thedrawings, the widths, lengths, thicknesses and the like of elements maybe exaggerated for convenience of illustration Like reference numeralsindicate like elements throughout the specification and drawings.

FIG. 1 is a circuit diagram illustrating an operational principle of alight emitting device having a plurality of light emitting cellsconnected in series according to an embodiment of the present invention.

Referring to FIG. 1, a first serial array 51 is formed by connectinglight emitting cells 51 a, 51 b and 51 c in series, and a second serialarray 53 is formed by connecting light emitting cells 53 a, 53 b and 53c in series.

Both ends of each of the first and second serial arrays 51 and 53 areconnected to an AC power source 55 and a ground, respectively. The firstand second serial arrays 51 and 53 are connected in parallel between theAC power source 55 and the ground. In other words, the is both ends ofthe first serial array 51 are electrically connected to those of thesecond serial array 53.

Meanwhile, the first and second serial arrays 51 and 53 are arrangedsuch that their light emitting cells are driven by currents flowing inopposite directions. In other words, as shown in the figure, anodes andcathodes of the light emitting cells included in the first serial array51 and anodes and cathodes of the light emitting cells included in thesecond array 53 are arranged in opposite directions.

Thus, if the AC power source 55 is in a positive phase, the lightemitting cells included in the first serial array 51 are turned on toemit light, and the light emitting cells included in the second serialarray 53 are turned off. On the contrary, if the AC power source 55 isin a negative phase, the light emitting cells included in the firstserial array 51 are turned off, and the light emitting cells included inthe second serial array 53 are turned on.

Consequently, the first and second serial arrays 51 and 53 arealternately turned on and off by the AC power source so that the lightemitting device including the first and second serial arrays 51 and 53continues to emit light.

Although light emitting chips each of which comprises a single lightemitting diode can be connected to one another to be driven by an ACpower source as in the circuit of FIG. 1, the space occupied by thelight emitting chips is increased. On the contrary, in the lightemitting device of the present invention, a single chip can be driven byconnecting it to an AC power source, thereby preventing an increase inthe space occupied by the light emitting device.

Meanwhile, although the circuit shown in FIG. 1 is configured such thatthe both ends of each of the first and second serial arrays 51 and 53are connected to the AC power source 55 and the ground, respectively,the circuit may be configured such that the both ends thereof areconnected to both terminals of the AC power source. Further, althougheach of the first and second serial arrays 51 and 53 has three lightemitting cells in FIG. 1, this is only an illustrative example forbetter understanding and the number of light emitting cells may beincreased, if necessary. The number of serial arrays may also beincreased.

FIG. 2 is a flow chart illustrating a method of fabricating a lightemitting device having a plurality of light emitting cells connected inseries according to an embodiment of the present invention. FIGS. 3 to 6are sectional views illustrating the method of fabricating the lightemitting device having the plurality of light emitting cells connectedin series according to the embodiment of the present invention.Meanwhile, FIG. 4 is a perspective view of FIG. 3.

Referring to FIGS. 2 and 3, a substrate 21 on which light emitting cellsare to be formed is provided (Step 1). The substrate 21 may be asubstrate made of sapphire (Al₂O₃), silicon carbide (SiC), zinc oxide(ZnO), silicon (Si), gallium arsenide (GaAs), gallium phosphorus (GaP),lithium aluminum oxide (LiAl₂O₃), boron nitride (BN), aluminum nitride(AlN) or gallium nitride (GaN). The substrate 21 is selected inconsideration of a lattice constant of a semiconductor layer to beformed thereon. For example, if a GaN based semiconductor layer isformed on the substrate 21, it is preferred that the substrate 21 be asapphire substrate.

A buffer layer 23 is formed on the substrate 21 (Step 3). The bufferlayer 23 is used to reduce stress generated due to a difference betweenthe lattice constants of the substrate 21 and the semiconductor layer tobe formed thereon. A semiconductor layer based on a III-N group such asAlN, GaN or InN may be used as the buffer layer 23. Here, the bufferlayer 23 is not additionally doped with impurities to prevent an excesscurrent from flowing through the buffer layer 23. The buffer layer 23may be formed using a hydride vapor phase epitaxy (HVPE) method, achemical vapor deposition (CVD) method or a metalorganic chemical vapordeposition (MOCVD) method. Thus, it is possible to form a thin bufferlayer 23.

Laminated rods 26 each of which has an n-type semiconductor rod 25, anactive layer 27 and a p-type semiconductor rod 29 sequentially laminatedare formed on the buffer layer 23 (Step 5). The laminated rods 26 may beformed using HYPE, CVD or MOCVD. At this time, as shown in FIG. 4, thelaminated rods 26 are spaced apart from one another on the buffer layer23.

The n-type semiconductor rod 25 may be formed of GaN doped withimpurities, e.g., silicon (Si), and the p-type semiconductor rod 29 maybe formed of GaN doped with impurities, e.g., magnesium (Mg). Meanwhile,the active layer 27 generally has a multi-layered film structure inwhich quantum well layers and barrier layers are repeatedly formed. Thequantum well layers and the barrier layers may be formed out of anAl_(x)In_(y)Ga_(1-x-y)N (0≦x, y≦1, 0≦x+y≦1) compound and have p-type orn-type impurities injected thereinto.

An insulating film 31 is formed on the substrate having the laminatedrods 26 to fill empty spaces between the adjacent laminated rods 26(Step 7). The insulating film 31 is formed of silicon oxide (SiO₂) bymeans of a chemical vapor deposition (CVD) method. After the siliconoxide film with a sufficient thickness is formed by means of thechemical vapor deposition method, the insulating film 31 is flatteneduntil top surfaces of the laminated rods 26 are exposed. As a result,the insulating film 31 for filling empty spaces is formed, and thelaminated rods 26 are exposed on a top surface of the insulating film31.

Referring to FIGS. 2 and 5, electrodes 33 may be formed on the exposedsurfaces of the laminated rods 26. The electrodes 33 can be formed byforming an electrode layer on the substrate having the insulating film31 formed thereon and patterning the electrode layer. The electrodes 33are formed on the p-type semiconductor rods 29 to form an ohmic contacttherewith.

Referring to FIGS. 2 and 6, the p-type semiconductor rods 29 and theactive layers 27 are etched, and the n-type semiconductor rods 25 areexposed. As a result, light emitting cells 26 a each of which includesthe n-type semiconductor rod 25, an etched active layer 27 a and anetched p-type semiconductor layer 29 a are formed (Step 11).

When the p-type semiconductor rod 29 and the active layer 27 are etched,the adjacent insulating film 31 may be etched. Further, the electrodes33 may be formed during this etching process. That is, after theelectrode layer is formed, the p-type semiconductor rods 29 and theactive layers 27 are sequentially etched together with the electrodelayer so that the electrodes 33 can be formed. In this case, theadditional patterning process for forming the electrode 33 may beeliminated.

Thereafter, metal wires 35 for electrically connecting the n-typesemiconductor rods 25 to the etched p-type semiconductor rods 29 a areformed (Step 13). The metal wires serially 35 connect the adjacent lightemitting cells 26 a to each other, thereby forming an array of seriallight emitting cells 26 a. The metal wires 35 connect the n-typesemi-conductor rods 25 to the electrodes 33. Further, before the metalwires 35 are formed, electrodes for ohmic contacts are additionallyformed on the exposed portions of the n-type semiconductor rods 25.Meanwhile, if the metal wires 35 form ohmic contacts with the p-typesemiconductor rod 29 a, the electrodes 33 may be eliminated.

The metal wires 35 can be formed by means of an air bridge method orstep-cover method. Meanwhile, to prevent the n-type semiconductor rod 25and the p-type semi-conductor rod 29 a, and the n-type semiconductor rod25 and the active layer 27 a in the same light emitting cell 26 a frombeing electrically connected directly to each other through the metalwire 35, a spacer (not shown) may be formed on side walls of the p-typesemiconductor rod 29 a and active layer 27 a.

At least two arrays of serial light emitting cells 26 a can be formed onthe substrate 21 through the metal wires 35, and the arrays areconnected to a power source so as to be driven by currents flowing inopposite directions. Accordingly, it is possible to fabricate a lightemitting device that can be connected directly to and then driven by anAC power source. Further, according to the embodiment of the presentinvention, since the laminated rods 26 spaced apart from one another areformed, it is possible to eliminate an etching process for electricallyseparating light emitting cells 26 a.

The structure of a light emitting device according to an aspect of thepresent invention will be described in detail below.

Referring again to FIG. 6, the light emitting device includes asubstrate 21. The substrate 21 may be a sapphire substrate. A bufferlayer 23 is located on the substrate 21. The buffer layer 23 may beformed of GaN using a metalorganic chemical vapor deposition (MOCVD)method.

A plurality of light emitting cells 26 a spaced apart from one anotherare located on the buffer layer 23. Each of the light emitting cells 26a comprises an n-type semi-conductor rod 25, an etched p-typesemiconductor rod 29 a, and an etched active layer 27 a interposedbetween the n-type semiconductor rod 25 and the p-type semiconductor rod29 a.

The n-type semiconductor rod 25 may be formed of GaN doped withimpurities, e.g., silicon (Si), and the p-type semiconductor rod 29 amay be formed of GaN doped with impurities, e.g., magnesium (Mg).Further, the etched active layer 27 a may have a multi-layered filmstructure in which quantum well layers and barrier layers are repeatedlyformed. The quantum well layers and the barrier layers may be formed outof an Al_(x)In_(y)Ga_(1-x-y)N (0≦x, y≦1, 0≦x+y≦1) compound and havep-type or n-type impurities injected thereinto.

Each of the active layer 27 a and the p-type semiconductor rod 29 a hasa smaller area as compared with that of the n-type semiconductor rod 25.Therefore, a portion of the n-type semiconductor rod 25 is exposed.Metal wires 35 connect the exposed n-type semiconductor rods 25 to thep-type semiconductor rods 29 a to form arrays of light emitting cells 26a connected in series.

As shown in the figure, although the light emitting cells 26 a can beconnected in series on a straight line, the present invention is notlimited thereto. The light emitting cells 26 a may be connected to oneanother in a zigzag arrangement.

Meanwhile, an electrode 33 may be interposed between the metal wire 35and the p-type semiconductor rod 29 a. The electrode 33 is in ohmiccontact with the p-type semi-conductor rod 29 a, thereby reducing thejunction resistance.

Further, an insulating film 31 may be interposed between the lightemitting cell 26 a and the metal wire 35. The insulating film 31 may bea silicon oxide film. In addition, a spacer (not shown) may beinterposed between the metal wire 35 and the etched p-type semiconductorrod 29 a and active layer 27 a. The spacer may be employed to separatethe metal wire 35 from the p-type semiconductor rod 29 a and the activelayer 27 a.

According to the embodiments of the present invention, there is provideda light emitting device that comprises light emitting diodes, which canbe connected directly to and driven by an AC power source. Therefore,the light emitting device can be used for the purpose of generalillumination using a household AC power source. Further, there isprovided a method of fabricating the light emitting device that can beconnected directly to and driven by the AC power source. According tothis method, it is possible to eliminate an etching process forseparating the light emitting cells.

What is claimed is:
 1. A light emitting device comprising: a substrate;a plurality of light emitting cells formed in rod shapes and spacedapart from one another on the substrate, each of the light emittingcells having an n-layer, an active layer and a p-layer; an electrodearranged on one of the n-layer or the p-layer; an insulating film; andwires connecting the spaced light emitting cells in series to form afirst serial array of light emitting cells, wherein at least one of then-layer and the p-layer is cylindrical rod-shaped, wherein the wires aredisposed on the insulating film, and a first wire contacts an outersurface of the insulating film and connects to the electrode, andwherein the wires comprise a step-cover structure.
 2. The light emittingdevice of claim 1, further comprising a buffer layer disposed betweenthe substrate and the plurality of light emitting cells, wherein thebuffer layer is formed of a III-N group material.
 3. The light emittingdevice of claim 1, wherein the rod-shaped light emitting cells areformed of a gallium nitride based compound.
 4. The light emitting deviceof claim 1, wherein the insulating film fills empty spaces between thelight emitting cells.
 5. The light emitting device of claim 1, furthercomprising: a plurality of second light emitting cells formed in rodshapes and spaced apart from one another on the substrate, each of thesecond light emitting cells having an n-layer, an active layer and ap-layer; and wires connecting the spaced second light emitting cells inseries to form a second serial array of second light emitting cells,wherein the first serial array of light emitting cells is configured tobe driven by a positive phase of an alternating current power source,and the second serial array of second light emitting cells is configuredto be driven by a negative phase of the alternating current powersource.
 6. The light emitting device of claim 1, wherein a portion ofthe first wire is arranged directly on the outer surface of theinsulating film along an extending direction of the first wire.
 7. Thelight emitting device of claim 6, wherein the portion of the first wireis arranged only on the outer surface of the insulating film.
 8. Thelight emitting device of claim 1, wherein a side surface of the firstwire contacts the outer surface of the insulating film.
 9. A method offabricating a light emitting device having a plurality of light emittingcells, the method comprising: forming a plurality of laminatedrod-shaped light emitting cells, each of the light emitting cells havingan n-layer, an active layer and a p-layer sequentially formed on asubstrate; etching the p-layers and the active layers until the n-layersare partially exposed; forming an insulating film; and connecting eachof the exposed portions of the n-layers to the p-layer of an adjacentlight emitting cell through wires in series or parallel after formingthe insulating film, wherein at least one of the n-layer and the p-layeris cylindrical rod-shaped, wherein an n-layer of a first light emittingcell is connected to a p-layer of a second light emitting cell via awire using a step-cover method, and wherein the wire is disposed on theinsulating film.
 10. The method of claim 9, forming a buffer layer onthe substrate, wherein an upper surface of the buffer layer comprisesfirst areas and a second area surrounding the first areas, and the lightemitting cells are formed directly on the upper surface of the bufferlayer in the first areas, respectively, and the insulating film isformed directly on the upper surface of the buffer layer in the secondarea.
 11. The method of claim 9, wherein each of the active layer andthe p-layer comprise a side surface that extends in a direction awayfrom the substrate, and the side surface of the active layer and theside surface of the p-layer are etched surfaces, wherein at least aportion of a side surface of the n-layer extending in a direction awayfrom the substrate is a non-etched surface.
 12. The method of claim 10,further comprising: exposing a portion of the light emitting cells;forming an electrode layer on the exposed portion of the light emittingcells; and forming an electrode on each of the light emitting cells bypatterning the electrode layer.
 13. The method of claim 10, furthercomprising: exposing a portion of the light emitting cells; forming anelectrode layer on the exposed portion of the light emitting cells; andsequentially patterning the active layer and the p-layer of theplurality of light emitting cells and the electrode layer, whereinpatterning the electrode layer forms an electrode on each of the lightemitting cells.
 14. The method of claim 13, further comprising: forminga metal wire to connect the n-layer of a first light emitting cell andthe p-layer of a second light emitting cell, wherein the insulating filmis interposed between the metal wire and side surfaces of each of thelight emitting cells.
 15. The method of claim 14, further comprising:forming at least two serial arrays of light emitting cells by connectingthe n-layer to the p-layer of adjacent light emitting cells; andconnecting the at least two serial arrays in inverse parallel, whereinthe light emitting device operates using an alternating current (AC)power source.